tree/: Convert flashchip decode range func ptr to enum

Replace the `decode_range` function pointer in `struct flashchip` to an
enum value. The enum value can be used to find the corresponding
function pointer by passing it to `lookup_decode_range_func_ptr()`.

Removing function pointers from flash chip data will make it possible to
represent the chip database in other formats that do not have to be
compiled together with the rest of flashrom's source files.

BUG=b:242479049
BRANCH=none
TEST=ninja && ninja test

Signed-off-by: Nikolai Artemiev <nartemiev@google.com>
Change-Id: If6d08d414d3d1ddadc95ca1d407fc87c23ab543d
---
M flashchips.c
M include/flash.h
M tests/chip_wp.c
M writeprotect.c
4 files changed, 73 insertions(+), 28 deletions(-)